1. Field of the Invention
The present invention relates generally to defibrillation methods, and more particularly, to an optimum truncated capacitive-pulse duration that is based upon the time constants of the system and of the heart.
2. Description of the Prior Art
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical direct current and voltage, in its most primitive form, goes back to the last century. [J. L. Prevost and F. Batelli, "Sur Quelques Effets des Descharges Electrriques sur 1e Couer des Mammifers", Comptes Rendus Hebdomadaires des Seances de L'Acadmie des Sciences, Vol. 129, p. 1267, 1899.] Because of the large currents required for defibrillation, large-area electrodes are employed. [A. C. Guyton and J. Satterfield, "Factors Concerned in Defibrillation of the Heart, Particularly Through the Unopened Chest", Am. J. of Physiology, Vol 167, p. 81, 195l.]
For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. The discharge of a capacitor C through a resistance R results in a curve of voltage versus time (and hence, of current versus time as well) that is a declining exponential function, with a characteristic time given by the product RC. But it has also been recognized for some time that the long-duration, low-amplitude "tail" of the capacitor-discharge pulse is detrimental. [J. C. Schuder, G. A. Rahmoeller, and H. Stoeckle, "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms", Circ Res., Vol. 19, p. 689, October, 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-discharge Defibrillation of Canine Ventricles", J. Applied Physiology, Vol 27, p. 480, October, 1969.] The exact reason for this detrimental effect is not known, although plausible speculations exist, with one possibility being that field heterogeneties cause arthythmias in significantly large regions of the heart. [P. S. Chen, et al., "The Potential Gradient Field Created by Epicardial Defibrillation Electrodes in Dogs", Circulation, Vol. 74, p. 626, September, 1986.] A convenient way to eliminate the low-amplitude "tail" of a capacitor discharge is by switching, which is to say, simply opening the capacitor-load circuit after a predetermined time, or else when voltage has fallen to a particular value. For this reason, the time-truncated capacitor discharge has been extensively used after its effectiveness was first demonstrated. [J. C. Schuder, et al., "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli", IEEE Trans. Biom Eng., Vol. BME-18, p. 410, November, 1971.]
The defibrillation effectiveness of time-truncated capacitor .discharges can be convincingly shown by comparing an untruncated waveform and a truncated waveform of equal effectiveness. The full discharge waveform 10 of FIG. 1A was generated by charging a 140-NF capacitor to 455 V, for an energy delivery of 30 J. But the truncated waveform 20 shown in FIG. 1B was equally effective for defibrillation in spite of having about only half the energy, and a lower initial voltage. This demonstration was carried through for the case of dogs using a catheter electrode and a subcutaneous patch [M. Mirowski, et al., "Standby Automatic Defibrillator", Arch Int. Med., Vol 126, p. 158, July, 1970], as well as with a dual-electrode intraventricular catheter. [J. C. Schuder, et al., "Ventricular Defibrillation in the Dog with a Bielectrode Intravascular Catheter", Arch. Int. Med., Vol. 132, p. 286, August, 1973.]The latter electrode arrangement was also used to demonstrate the point for the case of man. [M. Mirowski, et al., "Feasibility and Effectiveness of Low-energy Catheter Defibrillation in Man", Circulation, Vol 47, p. 79, January, 1973.] Such demonstrations that compact capacitor-storage systems could be used with effectiveness paved the way for implantable defibrillator system.
In spite of the dramatic results obtained with time-truncated capacitor-discharge defibrillator systems, the waveform specifications have not been systematically optimized. For example, some manufacturers such as Medtronic (in their PCD product) simply specify pulse duration, although the physician can choose and adjust the value. A typical value might be a programmable duration of 6 ms. Other manufacturers such as Cardiac Pacemakers (in their Ventak product) specify the relative amount of voltage decline at the time of truncation, with a typical value of the decline being 65% of the initial voltage. It has become customary to use the term "tilt" to describe the relative amount of such voltage decline, expressed either as a decimal fraction or a percentage. In algebraic language: EQU tilt=(V.sub.initial -V.sub.final)/V.sub.initial Eq. 1
Both of the systems just cited employ the monophasic waveform. This means that it consists of a single-polarity single pulse, specifically a time-truncated capacitor-discharge waveform like that of FIG. 1B. However, biphasic waveforms are also widely used. In this case capacitor discharge is also used, but instead of truncation, polarity reversal is accomplished (by switching once more), so that a second opposite-polarity pulse immediately follows the initial pulse, and is then itself truncated. The result is illustrated in FIG. 2.
Prior art in waveform specification for biphasic systems is parallel to that for monophasic systems. Specifically, some systems simply specify initial-pulse duration. [Baker, Intermedics, U.S. Pat. No. 4,821,732.] Other systems specify tilt. [Bach, Cardiac Pacemakers, U.S. Pat. No. 4,850,537.] The central focus of the present invention is to optimize pulse duration by using the model of this invention, which comprehends both the time constant of the system (capacitor and load resistance), and the natural time constant of the heart as explained below.
It is worthwhile to examine specific examples of prior-art waveform specification, In FIG. 3A is shown the simple pulse-duration specification, applicable to either monophasic pulses or biphasic initial pulses. And in FIG. 3B is shown a tilt specification, also applicable to either monophasic pulses or biphasic initial pulses.
The foundation for optimizing the time-truncated waveform is a family of mathematical neurophysiological models for tissue stimulation going back to the turn of the century, with the first important such model having been developed by Weiss. [G. Weiss, "Sur 1a Possibilite de Rendre Comparable entre Eux les Appareils Suivant a l'Excitation Electrique" Arch. Ital. deBiol., Vol. 35, p. 413, 1901.] He employed the ballistic-rheotome technique for pulse generation, wherein a rifle shot of known velocity is used to cut two wires in sequence, their spacing being set and measured. Cutting the first wire eliminated a short from a dc source, causing current to flow through the tissue under test, and cutting the second wire opened the circuit, terminating the pulse applied. Converting the electrical data into charge delivered by the pulse, Weiss found that the charge Q needed for stimulation was linearly dependent on pulse duration, d.sub.p. Specifically, EQU Q=K.sub.1 +K.sub.2 d.sub.p Eq. 2
Subsequently and similarly, the physiologist L. Lapicque collected substantial amounts of data on the amount of current required for tissue stimulation, using constant-current pulses of various durations. [L. Lapicque, "Definition Experimentelle de l'excitabilite," Proc. Soc. deBiol., Vol 77, p. 280, 1909.] Lapicque established an empirical relationship between the current I and the pulse duration d.sub.p, having the form: EQU I=K.sub.1 +(K.sub.2 /d.sub.p) Eq. 3
(Note that multiplying this expression through by d.sub.p yields an expression in charge rather than current, identically the equation given by Weiss. Thus, K.sub.1 =k.sub.1 /d.sub.p and K.sub.2 =k.sub.2 d.sub.p.)
Equation 3 of Lapicque shows that the necessary current and the pulse duration are related by a simple hyperbola, shifted away from the origin by the amount of the constant term K.sub.1. Hence the stimulating current required in a pulse of infinite duration is K.sub.1, a current value Lapicque termed the rheobase. Shortening the pulse required progressively more current, and the pulse length that required a doubling of current for excitation, or 2K.sub.1, he termed the chronaxie, d.sub.c. Substituting 2K.sub.1 and d.sub.c into Eq. 3 in place of I and d.sub.p, respectively, yields: EQU d.sub.c =K.sub.2 /K.sub.1 Eq. 4
For the sake of specific illustration, assume a rheobase current of 3.7 amperes, and a chronaxie time of 6 milliseconds. Then a plot of current strength required versus the pulse duration that must accompany it is as shown in FIG. 4.
Lapicque's model described cell stimulation, rather than defibrillation, but Bourland demonstrated that defibrillation thresholds in dogs and ponies followed the Lapicque model, provided average current is used in the exercise. [J. D. Bourland, W. Tacker, and L. A. Geddes, "Strength-Duration Curves for Trapezoidal Waveforms of Various Tilts for Transchest Defibrillation in Animals", Med Instr , Vol. 12, p. 38, 1978.] In a companion paper, the same workers showed that average current is a useful and consistent measure of defibrillation effectiveness for time-truncated pulses of a fixed duration through a substantial range of durations, from 2 to 20 milliseconds; in other words, so long as the exponential "tail" is eliminated, pulse effectiveness is not very dependent upon waveform details. [J. D. Bourland, W. Tacker, and L. A. Geddes, "Comparative Efficacy of Damped Sine Waves and Square Wave Current for Transchest Defibrillation in Animals", Med Instr., Vol. 12, p. 42, 1978]
The defibrillation chronaxie for the heart is usually between 2 ms and 4 ms, as can be seen in the chart of FIG. 5. (A journal citation for each entry is given below the chart.) In this synopsis of published data, chronaxie was inferred from a strength-duration curve such as that of FIG. 4 when such a curve was provided, and these cases are labeled "given"; in the case labeled "determined", chronaxie was calculated from discrete data provided. In the only other case (6. Geddes, et al. ), curves were given for waveforms of various tilts, and these were averaged to arrive at 2.8 ms. For the overall chart, 2.7.+-.0.9 ms is the average chronaxie value.